Managed pressure drilling in the most general sense is a process for drilling wellbores through subsurface rock formations in which wellbore fluid pressures are maintained at selected values while using drilling fluid that is less dense than that needed to produce a hydrostatic fluid pressure sufficient to prevent fluid entry into the wellbore from permeable rock formations as a result of naturally-occurring fluid pressure. Sufficient equivalent hydrostatic pressure to prevent fluid entry is provided in managed pressure drilling as a result of pumping drilling fluid at a selected rate through a drill string to increase its equivalent hydrostatic pressure in the wellbore, and by selectively controlling the rate of discharge of fluid from the wellbore annulus (the space between the wellbore wall and the exterior of the drill string). One such method and system are described in U.S. Pat. No. 6,904,981 issued to van Riet and commonly owned with the present disclosure. Generally, the system described in the van Riet '981 patent (called a “dynamic annular pressure control” or “DAPC” system) uses a rotating diverter or rotating control head to close the annular space between the drill string and the wellbore wall at the top of the wellbore. Fluid flow out of the wellbore is automatically controlled so that the fluid pressure gradient in the wellbore is maintained at a selected amount. That is, the actual fluid pressure at any selected vertical depth in the wellbore is controlled by the same process of selective pumping fluid into the wellbore and controlling discharge from the wellbore.
Certain types of marine drilling platforms float on the water surface, e.g., semisubmersible rigs and drill ships. Such drilling platforms are subject to a change in the elevation of the platform with respect to the bottom of the body of water in which a wellbore is being drilled due to wave and tide action. In order to maintain selected axial force on the drill bit during drilling operations, among other operations, it is necessary to adjust the elevation of the drilling equipment on the floating platform or corresponding operation. An example of a heave motion compensator is described in U.S. Pat. No. 5,894,895 issued to Welsh.
Heave motion compensation changes the effective length of both the drill string and the drilling fluid return line; therefore, managed pressure drilling systems, such as the one described in the van Riet '981 patent, may operate incorrectly on floating drilling platforms because the pressure measurements made by such managed pressure drilling systems infer the wellbore fluid pressure and fluid pressure gradient at any depth in the well from measurements of pressure made proximate the wellbore fluid outlet. Thus, a change in the length of the fluid return path along the wellbore will change the calculated wellbore annulus pressure.
In view of the foregoing, there is a need for a managed pressure drilling system operating method and arrangement that properly accounts for heave motion compensation on floating drilling platforms.